Ring signal generators generally fall into one of two classes. The first class uses a step up transformer to convert a low voltage, high current signal to a high voltage ring signal. The second class includes generators which use a high voltage external power supply as an input voltage source. Pulse width modulators or comparable devices are often employed in both classes of ring signal generators.
The first class of ring signal generator is shown by exemplary U.S. Pat. No. 4,220,826, which discloses a ring generator employing a variable pulse width modulator controlling a driver circuit. The output of the driver circuit controls a switch connected to the primary of a step up transformer. The output of the transformer's secondary winding is rectified and output through a pair of switches to produce an AC ring signal. A comparator is employed to develop an error signal which is used to control the output frequency of the pulse width modulator by controlling the duty cycle. On the other hand, U.S. Pat. No. 4,964,158 discloses a power supply for a telephone exchange employing an extremely small output power transformer while U.S. Pat. No. 4,015,091 discloses a telephone ring generator using a DC-to-DC converter controlled by an oscillator. The oscillator in the latter patent is switched between ON and OFF states by a pair of transistors, thereby controlling the output of the converter to provide 20 Hz power to the ringer.
The second class of ring signal generators is illustrated by exemplary U.S. Pat. No. 4,349,703, which discloses a programmable ring signal generator including a pulse width modulator driving a commutator switch via a pulse limiter and a DC-to-DC converter. Power to drive the ringer is provided by the commutator switch. Another example is found in U.S. Pat. No. 4,500,844, which discloses a ring signal generator employing delta modulation to generate ring signals with positive and negative offsets simultaneously. Delta modulated power amplifiers are used in the latter patent to produce the 240 volts peak-to-peak ring reference signals.
Other types of ring signal generators are disclosed in U.S. Pat. Nos. 4,239,935, 4,340,785 and 4,631,361. U.S. Pat. No. 4,239,935, for example, discloses a ring signal generator employing a trapezoidal waveform, the shape of which is controlled by a capacitor and two constant current sources. A voltage supply producing 200 volts is connected as the ring signal power source. On the other hand, U.S. Pat. No. 4,340,785 discloses a ring signal supply receiving a low level reference signal from a remote location, which is used to control the output of an inverter coupled to a low pass filter.
U.S. Pat. No. 4,631,361 discloses a ring generator which programmably produces DC and AC ring reference signals. The AC and DC signals can be specified independently. In particular, the ring generator employs a power switching amplifier controlled by a microcomputer.
Ring signal generators employing transformers, such as those described above, make it difficult to reduce the overall package size of the device. On the other hand, ring signal generators, such as those discussed in connection with the second class, employ devices which generally produce large amounts of heat, making it difficult to reduce the overall package size because heat sinks and similar devices must be included in the device.
The present invention was motivated by a desire to produce a ring signal generator which is small in size and high in efficiency.